Processes for Producing Synthetic Pyrite

ABSTRACT

Process of making high purity, synthetic FeS 2 , and an electrochemical battery employing such synthetic FeS 2  in the positive electrode. Synthetic FeS 2  may be prepared by a sulfidation process comprising reacting ferric oxide, hydrogen sulfide, and elemental sulfur at a temperature above the melting point of element sulfur. Synthetic FeS 2  may also be produced by a milling process that comprises (i) milling iron powder and sulfur powder in the presence of a milling media and a processing agent to provide a homogenous powder mixture, and (ii) treating the powder mixture to form FeS 2 . In the milling process, the powder mixture may be treated to form FeS 2  by heating the powder mixture or subjecting the powder mixture to a subsequent milling operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/236,857, filed Sep. 24, 2008 which claims the benefit of ProvisionalPatent Application No. 60/975,973 filed on Sep. 28, 2007, both of whichare fully incorporated by reference.

FIELD OF THE INVENTION

The present invention provides one or more processes for producingsynthetic iron disulfide (FeS₂), and particularly FeS₂ having a pyritecrystal structure. The present invention also provides a cathodecomprising synthetic FeS₂ and an electrochemical battery cell comprisingsuch a cathode.

BACKGROUND OF THE INVENTION

Lithium batteries (batteries containing metallic lithium as the negativeelectrode active material) are becoming increasingly popular as portablepower sources for electronic devices that have high power operatingrequirements. Common consumer lithium batteries includelithium/manganese dioxide (Li/MnO₂) and lithium/iron disulfide (Li/FeS₂)batteries, which have nominal voltages of 3.0 and 1.5 volts per cell,respectively.

Battery manufacturers are continually striving to design batteries withmore discharge capacity. This can be accomplished by minimizing thevolume in the cell taken up by the housing, including the seal and thevent, thereby maximizing the internal volume available for activematerials. However, there are practical limitations on the maximuminternal volume. For example, the Li/FeS₂ electrochemical system resultsin a volume increase upon discharge and the formation of reactionproducts. Thus, cell designs should incorporate sufficient void volumeto accommodate volume increases.

Another approach to increasing discharge capacity is to modify theinternal cell design and materials. How to best accomplish this candepend at least in part on the discharge requirements of the devices tobe powered by the batteries. For devices with low power requirements,the quantity of active materials tends to be very important, while fordevices with high power requirements, discharge efficiencies tend to bemore important. Lithium batteries are often used in high power devices,since they are capable of excellent discharge efficiencies on high powerdischarge.

In general, battery discharge efficiency decreases rapidly withincreasing discharge power. Therefore, for high power, providing highdischarge efficiency is a priority. This often means using designscontaining less active materials, thus sacrificing capacity on low powerand low rate discharge. For example, high interfacial surface areabetween the negative electrode (anode) and the positive electrode(cathode) relative to the volume of the electrodes is desirable toachieve good high power discharge efficiency. This is often accomplishedby using a spirally wound electrode assembly, in which relatively long,thin electrode strips are wound together in a coil. Unless the electrodecompositions have a high electrical conductivity, such long, thinelectrodes typically require a current collector extending along much ofthe length and width of the electrode strip. The high interfacialsurface area of the electrodes also means that more separator materialis needed to electrically insulate the positive and negative electrodesfrom each other. Because the maximum external dimensions are often setfor the cells, either by industry standards or the size and shape of thebattery compartments in equipment, increasing the electrode interfacialsurface area also means having to reduce the amount of active electrodematerials that can be used.

Reducing cell active material inputs in order to maximize high powerperformance is less desirable for batteries that are intended for bothhigh and low power use than for batteries intended for only high poweruse. For example, AA size 1.5 volt Li/FeS₂ (FR6 size) batteries areintended for use in high power applications such as photoflash anddigital still camera as well as general replacements for AA size 1.5volt alkaline Zn/MnO₂ batteries, which are often used in lower powerdevices. In such situations it is important to maximize both high powerdischarge efficiency and cell input capacity. While it is generallydesirable to maximize the electrode input capacity in any cell, therelative importance of doing so is greater in cells for lower powerusage.

To maximize the active material inputs in the cell and mitigate theeffects thereon of increasing the electrode interfacial surface area, itmay be desirable to use separator materials that take up as littleinternal volume in the cell as possible. There are, however, practicallimitations to doing so. The separator should be able to withstand thecell manufacturing processes without damage. The separator should alsoprovide adequate electrical insulation and ion transport between theanode and cathode and, desirably, do so without developing defectsresulting in internal short circuits between the anode and cathode whenthe cell is subjected to both normal and anticipated abnormal conditionsof handling, transportation, storage and use.

Separator properties can be modified in a number of ways to improve thestrength and resistance to damage. Examples are disclosed in U.S. Pat.Nos. 5,952,120; 6,368,742; 5,667,911 and 6,602,593, which are eachincorporated herein by reference in their entirety. However, changesmade to increase strength can also adversely affect separatorperformance based on factors such as, for example, cell chemistry,electrode design and features, cell manufacturing process, intended celluse, anticipated storage and use conditions, etc.

For certain cell chemistries, maximizing the amounts of active materialsin the cell can be more difficult. In lithium batteries, when the activecathode material reacts with the lithium to produce reaction productshaving a total volume greater than that of the reactants, swelling ofthe electrode assembly creates additional forces in the cell. Theseforces can cause bulging of the cell housing and short circuits throughthe separator. A possible solution to these problems includes usingstrong (often thicker) materials for the cell housing and inertcomponents within the cell. Using thicker materials, however, furtherlimits the internal volume available for active materials in cells withsuch active materials compared to cells with lower volume reactionproducts. For Li/FeS₂ cells, another possible solution, disclosed inU.S. Pat. No. 4,379,815, is to balance cathode expansion and anodecontraction by mixing another active material with the FeS₂. Such activecathode materials include CuO, Bi₂O₃, Pb₂Bi₂O₅, Pb₃O₄, CoS₂, andmixtures thereof. However, adding other active materials to the cathodemixture can affect the electrical and discharge characteristics of thecell.

Just as battery manufacturers are continually trying to improvedischarge capacity, they are also continually working to improve otherbattery characteristics, such as safety and reliability; making cellsmore resistant to internal short circuits can contribute to both. As isclear from the above discussion, changes made to improve resistance tointernal short circuits can be counterproductive in maximizing dischargecapacity.

The pyrite or iron disulfide (FeS₂) particles utilized inelectrochemical cell cathodes are typically derived from natural orewhich is crushed, heat treated, and dry milled to a particle size of 20to 30 microns. The fineness of the grind is limited by the reactivity ofthe particles with air and moisture. As the particle size is reduced,the surface area thereof is increased and is more susceptible toweathering. Weathering is an oxidation process in which the irondisulfide reacts with moisture and/or air to form iron sulfates. Theweathering process results in an increase in acidity and a reduction inelectrochemical activity. Small pyrite particles can generate sufficientheat during oxidation to cause hazardous fires within the processingoperation. Iron disulfide particles that have been utilized in cells canhave particles sizes that approach the final cathode coating thicknessof about 80 microns due to the inconsistencies of the dry millingprocess.

The dry milling process of iron disulfide is typically performed by amining company or an intermediate wherein large quantities of materialare produced. The processed iron disulfide is shipped and generallystored for extended periods of time before it can be used by the batteryindustry. Thus, during the storage period, the above-noted oxidation andweathering occur and the material degrades. Moreover, the large irondisulfide particle sizes can impact processes such as calendering,causing substrate distortion, coating to substrate bond disruption, aswell as failures from separator damage.

Pyrite particles derived from natural ores also contain a number ofimpurities. In particular, natural pyrite typically contains metal-basedimpurities containing metals such as Si, Mn, Al, Ca, Cu, Zn, As, and Co.Impurities are believed to decrease inputs and contribute to problemssuch as internal shorting and other defects in batteries. Some of theimpurities are soluble in the non-aqueous electrolyte and deposit on thenegative electrode as dendrites. The total concentration of variousimpurities in natural pyrite ore varies from lot to lot, and is often atleast about 3% by weight.

Synthetic pyrite has been manufactured, and may be produced having anaverage particle size less than 5 μm and even may be produced with anaverage particle size on the order of tens of nanometers. Whilesynthetic pyrite can be produced with little or no metal-basedimpurities as found in natural pyrite, synthetic pyrites typicallycontain iron sulfides having forms other than FeS₂. For example,synthetic pyrite may also contain iron sulfide (FeS). Iron sulfideimpurities in pyrite may also be represented as FeS, Fe_(1−y)S (wherey=0 to 0.2), and/or FeS_(1.3). As used herein, FeS encompasses FeS,Fe_(1−y)S, FeS_(1.3), and the like. FeS species are lower voltagematerials as compared to FeS₂ and may affect the discharge capacitiesand/or rate capability of Li/FeS₂ cells.

SUMMARY OF THE INVENTION

The present invention provides methods/processes for forming highpurity, synthetic iron disulfide (FeS₂). The processes provide syntheticFeS₂ that has reduced levels or is substantially free of impurities thatcan affect the electrical performance of Li/FeS₂ cells. The processesprovide FeS₂ that may have less than 1% by weight of metal impuritiesand/or less than 1% by weight of other impurities such as FeSimpurities.

The processes may provide synthetic FeS₂ particles ranging in size froma few microns down to tens of nanometers and can provide FeS₂ having arelatively large surface area.

In one aspect, the present invention provides a sulfidation process forproducing synthetic FeS₂ that comprises reacting ferric oxide (Fe₂O₃),elemental sulfur, and hydrogen sulfide (H₂S) to form FeS₂. The methodmay be carried out above the melting point of sulfur. The process may becarried out, for example, at a temperature of from about 125° C. toabout 400° C.

The sulfidation process may provide nano-FeS₂ having an average particlesize of from about 5 to about 200 nm. Larger particle sizes may beobtained at higher reaction temperatures. Further, the average particlesize may be increased by sintering the particles at a temperature in therange of from about 400° C. to below about 740° C. Sintering may be usedto increase the particle size from tens of nanometers to several hundrednanometers and even up to about 1 to about 5 μm.

In one embodiment, a method of forming synthetic FeS₂ comprises reactingFe₂O₃, elemental sulfur, and hydrogen sulfide in an inert atmosphere,the reaction being conducted at a temperature of from about 125° C. toabout 400° C. for a sufficient period of time to form synthetic FeS₂particles.

Unlike many other synthetic processes for making FeS₂, the sulfidationprocess in accordance with the present invention provides a process forforming high purity FeS₂ that may be carried out at relatively lowtemperatures. Depending on the sample size, the sulfidation process mayalso be a relatively fast process. Further, the sulfidation processprovides a clean method for making FeS₂ because solvents are notrequired and the reaction does not produce by-products that must beremoved or separated from the FeS₂. Generally, the only by-product iswater, but this is typically driven off as a gas during the process.

In another aspect, the present invention provides a method for producingsynthetic FeS₂ comprising intimately mixing iron powder and sulfurpowder in the presence of a process control agent and a milling media toform a substantially homogenous iron/sulfur powder mixture. Annealingthe powder mixture to form FeS₂ may be accomplished by heating thepowder mixture at a temperature of from at least about 400° C. to belowto about 740° C. The FeS₂ produced by milling iron and sulfur powdersand treating the resulting mixture may have some porosity (or voidvolume).

In still another aspect, the present invention provides a method offorming FeS₂ comprising performing a first milling operation comprisingintimately mixing iron powder and sulfur powder in the presence of aprocess control agent and a milling media to form a substantiallyhomogeneous powder mixture; removing the process control agent; andperforming a second milling operation comprising milling the homogeneouspowder mixture for a sufficient period of time to form FeS₂.

Synthetic FeS₂ produced by the methods/processes in accordance with thepresent invention may be used as an active material in a positiveelectrode, which may be used in an electrochemical battery cell.

In one aspect, the present invention provides a cathode comprising ahigh purity, synthetic FeS₂, such as the FeS₂ produced by one or more ofthe methods described herein. The present invention also provides anelectrochemical battery cell comprising such a cathode.

High purity, synthetic FeS₂ prepared by processes in accordance with thepresent invention also provides a useful control material to evaluatethe effects of different active materials or metal dopant on theperformance of Li/FeS₂ cells to be evaluated. By providing high purityFeS₂ that is substantially free of metal-based impurities and FeSimpurities, it is possible to formulate (cathode) compositions havingdesired and/or controlled amounts of other active materials or metaldopants and evaluate how such materials and/or concentrations of suchmaterials affect the performance of the cathode and/or battery cells.This cannot be done with natural pyrite where the purity level variesfrom lot to lot.

These and other features of the present invention will become apparentfrom the following detailed description in conjunction with the attachedfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, including other features and advantages thereof, may bebetter understood with reference to the detailed description and thefigures.

FIG. 1 is an embodiment of an electrochemical cell in accordance withthe invention;

FIG. 2 is an x-ray diffraction (XRD) pattern of synthetic FeS₂ producedby a comparative synthetic process;

FIG. 3 is a XRD pattern of synthetic FeS₂ produced by a sulfidationprocess in accordance with the present invention;

FIG. 4 illustrates a SEM micrograph at 20,000 times magnification ofsynthetic FeS₂ particles produced utilizing a sulfidation process inaccordance with the present invention;

FIG. 5 illustrates a field emission SEM micrograph at 200,000 timesmagnification of synthetic FeS₂ particles produced utilizing asulfidation process in accordance with the present invention;

FIG. 6 is a XRD pattern of synthetic FeS₂ prepared by a sulfidationprocess in accordance with the present invention in which the FeS₂particles are sintered;

FIG. 7 is a voltage discharge profile comparing the voltage dischargecharacteristics of natural pyrite to the voltage dischargecharacteristics of synthetic FeS₂ prepared utilizing a sulfidationprocess in accordance with the present invention;

FIG. 8 is a discharge profile comparing the specific discharge capacity,at different currents, of natural pyrite and synthetic FeS₂ prepared bya sulfidation process in accordance with the present invention;

FIG. 9 is a discharge profile comprising Li/FeS₂ cells using naturalFeS₂ or synthetic FeS₂ prepared by a sulfidation process in accordancewith the present invention, with the cells being discharged undercurrent densities of 20 mA/g and 200 mA/g;

FIG. 10 is a graph comparing the specific energy density of natural FeS₂to the synthetic FeS₂ from Example 2;

FIG. 11 is a XRD pattern of synthetic FeS₂ prepared by a milling processin accordance with the present invention;

FIG. 12 is a SEM image of FeS₂ particles produced by a milling processin accordance with the present invention; and

FIG. 13 is a SEM image of a cross section of FeS₂ particle produced by amilling process in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Unless otherwise specified, as used herein the terms listed below aredefined as follows:

Active material—one or more chemical compounds that are part of thedischarge reaction of a cell and contribute to the cell dischargecapacity, including impurities and small amounts of other moietiespresent.

Active material mixture—a mixture of solid electrode materials,excluding current collectors and electrode leads, that contains theelectrode active material.

Agglomerate—a collection of discrete particles bound together or acollection of discrete crystallites bound together.

Average particle size—the mean diameter of the volume distribution of asample of a composition (MV). Average particle size can be measured byany suitable method. An example of a suitable method includes using aMicrotrac Honeywell Particle Size Analyzer Model X-100 equipped with aLarge Volume Recirculator (LVR) (4 L Volume) Model 9320. The measuringmethod utilizes sonification to break up agglomerates and preventre-agglomeration. A sample of about 2.0 grams is weighed and placed intoa 50 ml beaker. 20 ml of deionized water and 2 drops of surfactant (1%Aerosol OT solution prepared from 10 ml 10% Aerosol OT available fromFisher Scientific in 100 mls deionized water with the solution beingwell mixed). The beaker sample solution is stirred, such as with astirring rod. The Large Volume Recirculator is filled to level withdeionized water and the sample is transferred from the beaker to theRecirculator bowl. A wash bottle is used to rinse out any remainingsample particles into the Recirculator bowl. The sample is allowed torecirculate for one minute before measurements are started. Thefollowing parameters are input for FeS₂ particles: TransparentParticles—No (absorbing); Spherical Particles—No; Fluid RefractiveIndex—1.33; Run Time—60 seconds. It will be appreciated by those skilledin the arts that the above method may not be suitable for evaluatingnano-size materials and that other methods may be used to evaluate theparticle size of nano-sized materials.

Capacity, discharge—the actual capacity delivered by a cell duringdischarge, generally expressed in amp-hours (Ah) or milliamp-hours(mAh).

Capacity, input—the theoretical capacity of an electrode, equal to theweight of each active material in the electrode times the theoreticalspecific capacity of that active material, where the theoreticalspecific capacity of each active material is determined according to thefollowing calculation: 1 [(96,487 ampere-seconds/mole)/(number ofgrams/mole of active material)]×(number of electrons/mole of activematerial)/(3600 seconds/hour)×(1000 milliampere hours/ampere-hour)(e.g., Li=3862.0 mAh/g, S=1672.0 mAh/g, FeS₂=893.6 mAh/g,CoS_(2871.3 mAh/g, CF) _(x)=864.3 mAh/g, Cu0=673.8 mAh/g, C₂F=623.0mAh/g, FeS=609.8 mAh/g, CuS=560.7 mAh/g, Bi₂O₃=345.1 mAh/g, MnO₂=308.3mAh/g, Pb₂Bi₂O₅=293.8 mAh/g and FeCuS₂=292.1 mAh/g).

Capacity, cell interfacial—the smaller of the negative and positiveelectrode capacity.

Capacity, electrode interfacial—the total contribution of an electrodeto the cell theoretical discharge capacity, based on the overall celldischarge reaction mechanism(s) and the total amount of active materialcontained within the portion of the active material mixture adjacent toactive material in the opposite electrode, assuming complete reaction ofall of the active material, generally expressed in Ah or mAh (where onlyone of the two major surfaces of an electrode strip is adjacent activematerial in the opposite electrode, only the active material on thatside of the electrode—either the material on that side of a solidcurrent collector sheet or that material in half the thickness of anelectrode without a solid current collector sheet—is included in thedetermination of interfacial capacity).

Crystallite—an entity containing a chemically homogeneous solid having arepeating, ordered atomic arrangement that coherently diffracts an X-raybeam.

Crystallite size—size of a crystallite as calculated using the ScherrerEquation.

Electrode assembly—the combination of the negative electrode, positiveelectrode, and separator, as well as any insulating materials,overwraps, tapes, etc., that are incorporated therewith, but excludingany separate electrical lead affixed to the active material, activematerial mixture or current collector.

Electrode gap—the distance between adjacent negative and positiveelectrodes.

Electrode loading—active material mixture dry weight per unit ofelectrode surface area, generally expressed in grams per squarecentimeter (g/cm²).

Electrode packing—active material dry weight per unit of electrodesurface area divided by the theoretical active material mixture dryweight per unit of electrode surface area, based on the real densitiesof the solid materials in the mixture, generally expressed as apercentage.

FeS₂ crystallite size—size of a FeS₂ crystallite as calculated using theScherrer Equation and the X-Ray diffraction peak width of the {200} ofpyrite in FeS₂.

Folded electrodes—electrode strips that are combined into an assembly byfolding, with the lengths of the strips either parallel to or crossingone another.

Interfacial height, electrode assembly—the average height, parallel tothe longitudinal axis of the cell, of the interfacial surface of theelectrodes in the assembly.

Interfacial volume, electrode assembly—the volume within the cellhousing defined by the cross-sectional area, perpendicular to thelongitudinal axis of the cell, at the inner surface of the containerside wall(s) and the electrode assembly interfacial height.

Nominal—a value, specified by the manufacturer, that is representativeof what can be expected for that characteristic or property.

Particle—a solid containing a single crystallite or two or morecrystallites chemically bound together.

Percent discharge—the percentage of the rated capacity removed from acell during discharge.

Room temperature—between about 20° C. and about 25° C.

Spiral wound electrodes—electrode strips that are combined into anassembly by winding along their lengths or widths, e.g., around amandrel or central core.

Void volume, electrode assembly—the volume of the electrode assemblyvoids per unit of interfacial height, determined by subtracting the sumof the volumes of the non-porous electrode assembly components and thesolid portions of the porous electrode assembly components containedwithin the interfacial height from the electrode assembly interfacialvolume (microporous separators, insulating films, tapes, etc. areassumed to be non-porous and non-compressible, and volume of a porouselectrode is determined using the real densities of the components andthe total actual volume), generally expressed in cm³/cm.

A battery cell in accordance with the invention has (i) an anodecomprising metallic lithium as the negative electrode active material,and (ii) a cathode comprising an active material comprising syntheticFeS₂. The anode and cathode may both be in the form of strips, which arejoined together in an electrode assembly to provide a high interfacialsurface area relative to the volumes of the electrodes containing activematerial. The higher the interfacial surface area, the lower the currentdensity and the better the cell's capability to deliver high power ondischarge. The cell also has a high ratio of cathode interfacialcapacity to electrode assembly interfacial volume. This means that thevolume of active materials in the electrode assembly is high, to providea high discharge capacity. The high volume of active materials can beachieved by controlling a number of variables, including:the ratio ofinterfacial input capacity to total input capacity, the volume of thecathode current collector, the concentration of active cathode materialin the cathode mixture, and the volume of separator in the electrodeassembly.

FIG. 1 shows an embodiment of a cell in accordance with the presentinvention. The cell 10 is an FR6 type cylindrical Li/FeS₂ battery cell.The cell 10 has a housing that includes a can 12 with a closed bottomand an open top end that is closed with a cell cover 14 and a gasket 16.The can 12 has a bead or reduced diameter step near the top end tosupport the gasket 16 and cover 14. The gasket 16 is compressed betweenthe can 12 and the cover 14 to seal an anode 18, a cathode 20 andelectrolyte within the cell 10. The anode 18, cathode 20 and a separator26 are spirally wound together into an electrode assembly. The cathode20 has a metal current collector 22, which extends from the top end ofthe electrode assembly and is connected to the inner surface of thecover 14 with a contact spring 24. The anode 18 is electricallyconnected to the inner surface of the can 12 by a metal tab (not shown).An insulating cone 46 is located around the peripheral portion of thetop of the electrode assembly to prevent the cathode current collector22 from making contact with the can 12, and contact between the bottomedge of the cathode 20 and the bottom of the can 12 is prevented by theinward-folded extension of the separator 26 and an electricallyinsulating bottom disc 44 positioned in the bottom of the can 12. Thecell 10 has a separate positive terminal cover 40, which is held inplace by the inwardly crimped top edge of the can 12 and the gasket 16.The can 12 serves as the negative contact terminal. Disposed between theperipheral flange of the terminal cover 40 and the cell cover 14 is apositive temperature coefficient (PTC) device 42 that substantiallylimits the flow of current under abusive electrical conditions. The cell10 also includes a pressure relief vent. The cell cover 14 has anaperture comprising an inward projecting central vent well 28 with avent hole 30 in the bottom of the well 28. The aperture is sealed by avent ball 32 and a thin-walled thermoplastic bushing 34, which iscompressed between the vertical wall of the vent well 28 and theperiphery of the vent ball 32. When the cell internal pressure exceeds apredetermined level, the vent ball 32, or both the ball 32 and bushing34, is forced out of the aperture to release pressurized gases from thecell 10.

The cell container is often a metal can with an integral closed bottom;though a metal tube that is initially open at both ends may also be usedinstead of a can. The can may be steel, that is plated with nickel on atleast the outside to protect the outside of the can from corrosion. Thetype of plating can be varied to provide varying degrees of corrosionresistance or to provide the desired appearance. The type of steel willdepend in part on the manner in which the container is formed. For drawncans the steel can be a diffusion annealed, low carbon, aluminum killed,SAE 1006 or equivalent steel, with a grain size of ASTM 9 to 11 andequiaxed to slightly elongated grain shape. Other steels, such asstainless steels, can be used to meet special needs. For example, whenthe can is in electrical contact with the cathode, a stainless steel maybe used for improved resistance to corrosion by the cathode andelectrolyte.

The cell cover is typically metal. Nickel plated steel may be used, buta stainless steel is often desirable, especially when the cover is inelectrical contact with the cathode. The complexity of the cover shapewill also be a factor in material selection. The cell cover may have asimple shape, such as a thick, flat disk, or it may have a more complexshape, such as the cover shown in FIG. 1. When the cover has a complexshape like that in FIG. 1, a type 304 soft annealed stainless steel withASTM 8-9 grain size may be used, to provide the desired corrosionresistance and ease of metal forming. Formed covers may also be platedwith any suitable material such as, for example, nickel.

The terminal cover should have good resistance to corrosion by water inthe ambient environment, good electrical conductivity and, when visibleon consumer batteries, an attractive appearance. Terminal covers areoften made from nickel plated cold rolled steel or steel that is nickelplated after the covers are formed. Where terminals are located overpressure relief vents, the terminal covers generally have one or moreholes to facilitate cell venting.

The gasket may be made from any suitable thermoplastic material thatprovides the desired sealing properties. Material selection is based inpart on the electrolyte composition. Examples of suitable materialsinclude, but are not limited to, polypropylene, polyphenylene sulfide,tetrafluoride-perfluoroalky-1 vinylether copolymer, polybutyleneterephthalate, and combinations thereof. Particularly suitable gasketmaterials include polypropylene (e.g., PRO-FAX® 6524 from BasellPolyolefins, Wilmington, Del., USA), polybutylene terephthalate (e.g.,CELANEX® PBT, grade 1600A from Ticona-US, Summit, N.J., USA) andpolyphenylene sulfide (e.g., TECHTRON® PPS from Boedeker Plastics, Inc.,Shiner, Tex., USA). Small amounts of other polymers, reinforcinginorganic fillers and/or organic compounds may also be added to the baseresin of the gasket.

The gasket may be coated with a sealant to provide the best seal.Ethylene propylene diene terpolymer (EPDM) is a suitable sealantmaterial, but other suitable materials can be used.

The vent bushing may be made from a thermoplastic material that isresistant to cold flow at high temperatures (e.g., 75° C.). Thethermoplastic material comprises a base resin such as, for example,ethylene-tetrafluoroethylene, polybutylene terephthlate, polyphenylenesulfide, polyphthalamide, ethylenechloro-trifluoroethylene,chlorotrifluoroethylene, perfluoroalkoxyalkane, fluorinatedperfluoroethylene polypropylene and polyetherether ketone. Particularlysuitable resins include ethylene-tetrafluoroethylene copolymer (ETFE),polyphenylene sulfide (PPS), polybutylene terephthalate (PBT), andpolyphthalamide. The resin can be modified by adding athermal-stabilizing filler to provide a vent bushing with the desiredsealing and venting characteristics at high temperatures. The bushingcan be injection molded from the thermoplastic material. TEFZEL® HT2004(ETFE resin with 25 weight percent chopped glass filler) is an exampleof a suitable thermoplastic material.

The vent ball can be made from any suitable material that is stable incontact with the cell contents and provides the desired cell sealing andventing characteristic. Glasses or metals, such as stainless steel, canbe used.

The anode comprises a strip of lithium metal, sometimes referred to aslithium foil. The composition of the lithium can vary, though forbattery grade lithium the purity is always high. The lithium can bealloyed with other metals, such as aluminum, to provide the desired cellelectrical performance. Battery grade lithium-aluminum foil containing0.5 weight percent aluminum is available from Chemetall Foote Corp.,Kings Mountain, N.C., USA.

The anode may have a current collector, within or on the surface of themetallic lithium. As in the cell in FIG. 1, a separate current collectormay not be needed, since lithium has a high electrical conductivity, buta current collector may be included, for example, to maintain electricalcontinuity within the anode during discharge, as the lithium isconsumed. When the anode includes a current collector, it may be made ofcopper because of its conductivity, but other conductive metals can beused as long as they are stable inside the cell.

A thin metal strip often serves as an electrical lead, or tab,connecting the anode to one of the cell terminals (the can in the caseof the FR6 cell shown in FIG. 1). The metal strip is often made fromnickel or nickel plated steel and affixed directly to the lithium. Thismay be accomplished by embedding an end of the lead within a portion ofthe anode or by simply pressing an end of the lead onto the surface ofthe lithium foil.

The cathode may be in the form of a strip that comprises a currentcollector and a cathode formulation that includes one or moreelectrochemically active materials, usually in particulate form. Thecathode formulation, which is typically a slurry, comprises syntheticiron disulfide (FeS₂) as an active material. The active material maycomprise greater than about 50 weight percent FeS₂. The active materialmay comprise at least 95 weight percent FeS₂, at least 99 weight percentFeS₂, and in one embodiment, FeS₂ is the sole active cathode material.In one embodiment, the FeS₂ of the active material comprises syntheticFeS₂. The FeS₂ of the active material may comprise a mixture ofsynthetic FeS₂ and FeS₂ derived from a natural ore. Alternatively, theFeS₂ of the active material may be comprised of only synthetic FeS₂.

The cathode can also contain one or more additional active materials,depending on the desired cell electrical and discharge characteristics.The additional active cathode material may be any suitable activecathode material. Examples of other active materials include, but arenot limited to, Bi₂O₃, C₂F, CF, (CF)_(n), CoS₂, CuO, CuS, FeS, FeCuS₂,MnO₂, Pb₂Bi₂O₅, S, or mixtures of two or more thereof.

The synthetic FeS₂ suitable for use in the active material may have apurity, on a metals basis, of at least about 97% and may be about 99% orhigher. As previously described, metal-based impurities may includemetals such as, but not limited to, Mn, Al, Ca, Cu, Zn, As, Co, and thelike. In one embodiment, the total concentration of metal impurities byweight of the synthetic FeS₂ is about 1% or less, in another embodimentabout 0.1% or less, and in another embodiment about 0.01% or less.

Desirably, the synthetic FeS₂ has a relatively low concentration of FeSimpurities. In one embodiment, the synthetic FeS₂ has a FeS content byweight of the FeS₂ of about 3% or less, in another embodiment about 1.0%or less, in another embodiment about 0.1% or less, and in anotherembodiment about 0.01% or less.

The synthetic FeS₂ may have a relatively small average particle size.Electrochemical cells prepared with FeS₂ particles having a reducedaverage particle size exhibit increased cell voltage at any given depthof discharge, irrespective of cell size. The synthetic FeS₂ particlesmay have an average particle size less than about 10 less than about 5μm, or less than about 3 μm. In one embodiment, the synthetic FeS₂, mayhave an average particle size in the range of from about 1 to about 5μm. The synthetic FeS₂ may even have an average particle size in thesub-micron range (<1 μm) range including, but not limited to, less thanabout 500 nm, less than about 250 nm, less than about 100 nm, even lessthan about 10 nm. In one embodiment, the synthetic FeS₂ may have anaverage particle in the range of from about 5 nm to about 200 nm.

Higher purity, synthetic FeS₂ may be provided by one or more of theprocesses in accordance with the present invention. In one embodiment,FeS₂ may be formed by a sulfidation process. In another embodiment,synthetic FeS₂ may be formed by a milling process. These processes arenow described in detail.

Sulfidation Process

In one embodiment, synthetic FeS₂ may be formed by a sulfidation processthat comprises reacting (i) ferric oxide (Fe₂O₃), (ii) elemental sulfur,and (iii) hydrogen sulfide (H₂S) for a sufficient period of time to formFeS₂. While not wishing to be bound by any theory, the reaction isbelieved to proceed as follows:

Fe₂O₃+3H₂S+⅛S₈→2FeS₂+3H₂O

The ferric oxide may be provided as nanoparticles, which may also bereferred to as “nano rust.” The ferric oxide particles may have aparticle size less than about 100 nanometers (nm). The ferric oxideparticles may have a particle size of from about 1 to about 100 nm; inone embodiment the ferric oxide particles have a particle size of fromabout 3 to about 50 nm; and in another embodiment the ferric oxideparticles have a particle size of from about 3 to about 10 nm.Applicants have found that if the ferric oxide particles are too large,the reaction may not go to completion to form FeS₂. Rather, if theferric oxide particles are too large, the resulting product may comprisean unreacted (Fe₂O₃) core having a FeS₂ coating on the outer surface.

The elemental sulfur component (ii) may be provided as a solid. Solidsulfur may be provided in any suitable form including, for example,molten sulfur. The size of the solid sulfur particles is notparticularly limited. In one embodiment, the sulfur may be provided asparticles having a particle size of about 1 to about 5 μm.

Hydrogen sulfide (H₂S) is provided as a gas. In one embodiment, the H₂Smay be provided as 100 volume percent of H₂S. In another embodiment, theH₂S may be provided as a volume of H₂S in a carrier gas. The carrier gasmay be an inert gas such as, for example, nitrogen (N₂). For example, inone embodiment, the H₂S may be provided as a gas comprising from about1% by volume to about 99% by volume of H₂S in a carrier gas such as N₂;in another embodiment from about 3% by volume to about 70% by volume;and in another embodiment from about 6% by volume to about 40% byvolume.

Applicants have found that FeS₂ may be obtained with the disclosedsulfidation process at relatively low temperatures, e.g., below about400° C. Generally, the reaction may be conducted at a temperature abovethe melting point of sulfur (about 113° C.) and below about 400° C. Inone embodiment, the reaction may be carried out at a temperature in therange of from about 125° C. to about 400° C.; and in another embodimentfrom about 125° C. to about 300° C. In another embodiment, the reactionmay be carried out at a temperature in the range of from about 125° C.to about 200° C. The temperature may be adjusted as desired to produceFeS₂ particles of different sizes, with larger particles being producedat higher reaction temperatures. Complete sulfidation to FeS₂ may berealized at relatively low temperatures such as from above the meltingpoint of sulfur to about 125° C. Further, at temperatures nearer to themelting point of sulfur (e.g., around 125° C.), the resulting FeS₂particles stay relatively small and have a relatively large surfacearea.

The reactants may be combined in about a 1:3:0.125 molar ratio ofFe₂O₃:H₂S:S₈. The sulfidation process may be carried out by firstreacting the Fe₂O₃ and elemental sulfur at a temperature above themelting point of sulfur for a selected period of time. After reactingthe Fe₂O₃ and sulfur for a desired period of time, the H₂S may beintroduced to the system and the reaction may proceed for a period oftime sufficient for complete sulfidation to occur. At a reactiontemperature of about 125° C., for example, complete sulfidation mayoccur in less than about five hours depending upon sample size. In oneembodiment, the Fe₂O₃ and the sulfur may be mixed at a firsttemperature, e.g., about 125° C., and the temperature may be increasedafter adding the H₂S. It will be appreciated that the reaction need notbe conducted at a single temperature. For example, the reaction may beheld at a first temperature for a selected period of time (e.g., 125°C.) and then a temperature ramp may be used to increase the temperatureat a desired rate to a second selected temperature (e.g., about 200°C.). As described above, higher temperatures may be desirable to providelarger FeS₂ particles.

It may be desirable to conduct the reaction in an inert atmosphere suchas, for example, argon, nitrogen, or the like. In one embodiment, theFe₂O₃ and sulfur may be charged to the system, and the system is flushedwith an inert gas at least prior to the addition of the H₂S.

The sulfidation process provides FeS₂ particles having a particle sizeof less than about 1 μm. The sulfidation process may provide particleshaving an average particle size of about 250 nm or less, about 200 nm orless, about 100 nm or less, even about 10 nm or less. In one embodiment,the FeS₂ particles may have an average particle size of about 200 nm(which may indicate that the majority of the distribution falls betweenabout 70 nm and about 600 nm). In one embodiment, the process providesFeS₂ particles having a particle size of from about 5 to about 600 nm;and in another embodiment from about 5 to about 200 nm. The FeS₂particles may have a crystallite size in the range of from about 5 toabout 100 nm. As described above, the particles size may be controlledby selecting the size of the starting Fe₂O₃ particles and/or thetemperature at which the reaction is run (with larger particles beingobtained at higher reaction temperatures).

If desired, larger FeS₂ particles may be produced by sintering the FeS₂particles obtained from the sulfidation process. Sintering may beaccomplished by heating the particles at a temperature in the range offrom about 400° C. to below the temperature at which FeS₂ decomposes(about 740° C.). For example, the FeS₂ particles may be sintered at atemperature in the range of from about 400° C. to about 700° C. for asufficient period of time to increase the particle size of the FeS₂particles. Typically, the sintering step should be carried out undervacuum at a pressure below atmospheric pressure and/or in an inertatmosphere. Sintering may be used to increase the crystallite size ofthe FeS₂ particles from tens of nanometers to the order of hundreds ofnanometers, or even several microns. For example, the FeS₂ particles mayhave a crystallite size of from about 35 nm to about 3 μm aftersintering. In one embodiment, FeS₂ particles obtained from thesulfidation process may be sintered at a temperature of from about400-500° C. to increase the particle size from tens of nanometers tofrom about 150-200 nm (the FeS₂ particles may have a crystallite size offrom about 35 to about 200 nm). Sintering may also be used to increasethe FeS₂ particle size from tens or hundreds of nanometers to about 1 toabout 3 μm. By heating at a temperature of about 700° C. under vacuum,for example, FeS₂ particles having a particle size of about 200 nm toabout 3 μm may be obtained from nano-sized particles.

FeS₂ particles produced by the sulfidation process may exhibit bothpyrite and marcasite crystal phases. The resulting FeS₂ product existsprimarily in the pyrite phase but may include traces of marcasitecrystals. While not wishing to be bound by any theory, it has been foundthat sintering the FeS₂ particles may also convert the marcasitecrystals to pyrite crystals. For example, the marcasite crystals may beconverted to pyrite crystals by heating the FeS₂ particles, such as bysintering the particles at a temperature above about 400° C. and belowabout 740° C. For example, the marcasite crystals may be converted topyrite crystals by sintering at a temperature of about 400° C. to about500° C.

The sulfidation process provides high purity synthetic FeS₂ particles.In one aspect, the synthetic FeS₂ has a high purity on a metals basis.The FeS₂ may have a purity, on a metals basis, greater than about 97%and desirably, having a purity of greater than about 99%. In oneembodiment, the total concentration of metal impurities by weight of thesynthetic FeS₂ is about 1% or less, in another embodiment about 0.1% orless, and in another embodiment about 0.01% of less.

The synthetic FeS₂ produced by the sulfidation process may also beconsidered as having a high purity on the basis of iron sulfide (FeS)impurities. The FeS₂ produced by the sulfidation process may have lessthan about 3% by weight or iron sulfide impurities, and desirably lessthan 1% by weight of iron sulfide impurities. In one embodiment, theFeS₂ has iron sulfide impurities of about 0.1% or less and in anotherembodiment about 0.01% or less.

Additionally, the synthetic FeS₂ produced by the sulfidation process mayalso be substantially free of oxide species, e.g., sulfates. In oneembodiment, the FeS₂ has less than about 3% by weight of oxide species.Nano-FeS₂ formed by the sulfidation process typically has a relativelylarge surface area (e.g., about 100 m²/g or higher) and may be moresusceptible to oxidation than natural pyrite or larger FeS₂ particles. Amono layer of oxygen on nano-sized FeS₂ could provide oxide impuritiesin the range of about 0 to about 10% by weight. Therefore, it may bedesirable to limit exposure of nano-FeS₂ particles to oxygen containingenvironments until they can be formulated into a cathode formulation orsintered to provide larger FeS₂ particles. For example, it may bedesirable to store the nano-FeS₂ particles in a dry box until they areto be formulated into a cathode formulation and/or prepare the cathodeformulation in a dry box.

The method may also include coating the FeS₂ particles with a protectivecoating material to reduce or prevent oxidation of the FeS₂ particles.In one aspect, the coating may be a temporary coating that isdissolvable in a cathode formulation environment. In another embodiment,the coating may be formed from a conductive material. Suitableconductive materials include, but are not limited to carbon materials,metal materials, metal oxides, and organic conductive materials.Suitable metal oxides include, for example, cobalt oxide, manganeseoxide, and the like. Suitable organic conductive materials include, forexample, polyphenylene derivatives. A particularly suitable conductivematerial for use in the coating layer is a carbon coating. The carbonmaterial may comprise, for example, acetylene black, graphite, carbonblack, mixtures of two or more thereof, and the like. The protectivecoating layer may be applied to the FeS₂ particles in any suitablemanner including spraying, dipping, brushing, and the like. In oneembodiment, the protective coating layer may be applied using spraypyrolysis. The thickness or coating weight of the protective coatinglayer may be selected as desired for a particular purpose or intendeduse, but should generally be sufficient to adequately protect the FeS₂particles against oxidation.

The sulfidation process is a relatively “clean” process and does notrequire additional separation or cleaning steps to obtain the final FeS₂product. When the process utilizes solid sulfur as a starting material,the process may be run without any solvents that would require removalor clean up. Further, the only other product of the sulfidation processis water (H₂O). The water, however, evaporates because the reaction iscarried out a temperature above the melting point of sulfur (about 113°C.), which is also above the boiling point of water.

Milling Process

In another embodiment, synthetic FeS₂ may be prepared by a processcomprising (i) mixing iron powder and sulfur powder to provide asubstantially homogenous iron/sulfur powder mixture, and (ii) treatingthe powder mixture under conditions sufficient to form FeS₂.

Mixing of the iron and sulfur powder may be accomplished by any suitabletechnique such as, for example, mechanical milling. Mechanical millingmay be accomplished using any suitable milling devices including, butnot limited to, roll mills, granulating mills, ball mills, media mills,bead mills, head mills, and the like. Milling and intimate mixing of theiron and sulfur powders may be accomplished using any suitable millingmedia including, but not limited to, steel, ceramic, glass, zirconiamedia, and the like. In one embodiment, the milling media issubstantially free of iron. Despite containing iron, steel shot isparticularly suitable as the milling media. The milling media may beprovided in any suitable amount as desired. For example, the weightratio of iron and sulfur powder to milling media may be, for example, inthe range of from about 1:4 to about 1:10, in the range of from about1:5 to about 1:10, or in the range of from about 1:7 to about 1:10. Inone embodiment, the weight ratio of iron and sulfur powder to millingmedia may be about 1:7.

The iron and sulfur powders are mixed in the presence of a processcontrol agent (which may also be referred to as a processing agent). Theprocess control agent is not particularly limited except that it shouldbe substantially free of oxygen. Suitable materials for the processcontrol agent include hydrocarbons such as, for example, alkanesincluding but not limited to pentane, heptane, hexane, octane, nonane,decane, combinations of two or more thereof, and the like. The processcontrol agent should be present in an amount sufficient to facilitateforming a homogenous powder mixture from the iron and sulfur powdersduring the milling process. If too little process control agent ispresent, the process control agent may be consumed by the powder(s), thepowders may agglomerate, and/or intimate mixing of the powders may notoccur (e.g., the powders may attach to the walls of the mixing vesselresulting in poor milling efficiency) such that a homogenous mixture isnot obtained. In one embodiment, the process control agent may bepresent in an amount of from about 5 to about 15 percent by weight ofthe total weight of the iron powder, sulfur powder, and milling media.In one embodiment, the process control agent is present in an amount offrom about 7 to about 10 percent by weight of the total weight of ironpowder, sulfur powder, and milling media.

Generally, the iron powder and sulfur powder should be present in atleast a 1:2 molar ratio of iron to sulfur (i.e., at least astoichiometric ratio of Fe to S to form FeS₂.) It may be desirable toprovide the sulfur powder in an amount in excess of that required by thestoichiometric ratio to ensure that a sufficient amount of sulfur ispresent to form FeS₂. For example, if steel is used as the millingmedia, the system may pick up some iron from the milling media, whichmay result in the formation of a small amount of FeS during thetreatment operation. If a slight excess of sulfur is used, the extrasulfur can react with the extra iron that may be present from themilling media.

The iron and sulfur powders may be mixed for a sufficient period of timeto provide a homogenous iron/sulfur powder mixture. It will beappreciated that the time for mixing may vary depending on the millingprocess used, the size of the system (e.g., the total amount of iron andsulfur powder), the concentration of milling media, the type of millingmedia, and the like, and may be readily ascertained by a person skilledin the art. In one embodiment, the iron and sulfur powders are mixed byball milling for a period of about five hours. To provide a high purity,synthetic FeS₂ product using a milling method, the mixing process shouldbe carried out under conditions that disfavor the formation ofbyproducts such as oxides and sulfides. Therefore, it may be desirableto carry out the mixing operation in an inert atmosphere such as, forexample, an argon atmosphere.

Following mixing of the iron and sulfur powder, the substantiallyhomogenous mixture is treated under sufficient conditions to form FeS₂.Typically, the process control agent is removed prior to treating thepowder mixture to form FeS₂. The process control agent may be removed byany suitable method including, for example, evaporation. In oneembodiment, the powder mixture is treated by annealing the powdermixture at a sufficient temperature for a sufficient period of time toform FeS₂. For example, FeS₂ may be formed by annealing the iron/sulfurpowder mixture at a temperature in the range of from at least about 400°C. to a temperature below the decomposition temperature of FeS₂ (about740° C.). In one embodiment, the powder mixture is annealed at atemperature of from about 450° C. to about 500° C. Heating may beaccomplished using a temperature ramp or gradient to reach the desiredannealing temperature. In one embodiment, the iron/sulfur powder mixtureis heated using a temperature ramp of from about 1 to about 3° C. perminute up to 450° C., and then holding the temperature 450° C. for aboutforty-five minutes. A heating ramp may be desirable to take thetemperature through the melting point of sulfur at a relatively slowrate to ensure that all of the sulfur reacts with the iron to form FeS₂(and avoid forming byproducts such as FeS). The rate of heating may beselected as desired to suit a particular need or purpose. For example,the temperature may be increased at a first rate through the meltingpoint of sulfur and then increased at a faster rate until the finaltemperature of heating is reached.

In another embodiment, the powder mixture is treated to form FeS₂ bysubjecting the powder mixture to a subsequent milling operation. Inparticular, after milling the iron and sulfur powder to form the powdermixture, the processing agent may be removed from the powder mixture,and the powder mixture may be milled to form FeS₂. The second millingoperation may be accomplished using any suitable milling methodincluding those described above.

FeS₂ formed by a milling method in accordance with the present inventionmay have an average particle size of from about 1 μm to about 10 μM.Additionally, the FeS₂ particles formed by a milling method inaccordance with the present invention may exhibit some porosity (andexhibit some void volume).

FeS₂ produced by the milling method has a purity, on a metals basis, ofat least about 97% and desirably has a purity of at least about 99%. Inone embodiment, the total concentration of metal impurities by weight ofthe synthetic FeS₂ is about 1% or less, in another embodiment about 0.1%or less, and in another embodiment about 0.01% of less. Additionally,FeS₂ produced by the milling method contains about 3% by weight or lessof FeS impurities and desirably about 1% by weight or less of FeSimpurities. In one embodiment, the FeS₂ has iron sulfide impurities ofabout 0.1% or less and in another embodiment about 0.01% or less.

Some process control agent may become entrained in the FeS₂ produced bythe milling method. The process control agent may become entrained inthe product from milling the iron and sulfur powders and/or during theannealing operation. More particularly, carbon from the process controlagent may become entrained in the FeS₂. The hydrogen atoms from thehydrocarbon process control agent may escape or be driven off during theannealing operation with carbon being left behind in the FeS₂. Withoutbeing bound by any particular theory, the carbon may be present in avariety of forms including, but not limited to, amorphous carbon,graphite, carbide, and solid solution in FeS₂.

The amount of carbon entrained in the FeS₂ may be a function of themilling time and the amount of process control agent used in milling theiron and sulfur powders. The amount of entrained carbon generallyincreases with longer milling times. For a given milling time, theamount of entrained process control agent (and, therefore, carbon)increases with a decreasing amount of process control agent added to theinitial charge of iron and sulfur powder.

The amount of carbon entrained in the FeS₂ may be about 1% or less byweight of the FeS₂. In one embodiment, the amount of carbon retained inthe powder is about 0.50% or less by weight of the FeS₂. In oneembodiment, the amount of carbon retained in the powder is about 0.25%or less by weight of the FeS₂. In one embodiment, the amount of carbonretained in the powder is about 0.15% or less by weight of the FeS₂.

If desired, additives or dopants could be added to the initial charge ofthe milling method to provide FeS₂ having a particular additive ordopant concentration. The dopant additive could be, for example, one ormore metals, graphite, or carbon black.

In addition to the active material, the cathode mixture typicallycontains other materials. For example, a binder is generally used tohold the particulate materials together and adhere the mixture to thecurrent collector. One or more conductive materials such as metal,graphite and carbon black powders may be added to provide improvedelectrical conductivity to the mixture. The amount of conductivematerial used can be dependent upon factors such as, for example, theelectrical conductivity of the active material and binder, the thicknessof the mixture on the current collector, the current collector design,and the like. Small amounts of various additives may also be used toenhance cathode manufacturing and cell performance. The following areexamples of active material mixture materials for Li/FeS₂ cell cathodes.Graphite: KS-6 and TIMREX® MX15 grades synthetic graphite from TimcalAmerica, Westlake, Ohio, USA. Carbon black: Grade C55 acetylene blackfrom Chevron Phillips Company LP, Houston, Tex., USA. Binder:ethylene/propylene copolymer (PEPP) made by Polymont Plastics Corp.(formerly Polysar, Inc.) and available from Harwick StandardDistribution Corp., Akron, Ohio, USA; non-ionic water solublepolyethylene oxide (PEO): POLYOX® from Dow Chemical Company, Midland,Mich., USA; and G1651 grade styrene-ethylene/butylenes-styrene (SEBS)block copolymer from Kraton Polymers, Houston, Tex. Additives: FLUO HT®micronized polytetrafluoroethylene (PTFE) manufactured by Micro PowdersInc., Tarrytown, N.Y., USA (commercially available from Dar-Tech Inc.,Cleveland, Ohio, USA) and AEROSIL® 200 grade fumed silica from DegussaCorporation Pigment Group, Ridgefield, N.J.

The current collector may be disposed within or imbedded into thecathode surface, or the cathode mixture may be coated onto one or bothsides of a thin metal strip. Aluminum is a commonly used material. Thecurrent collector may extend beyond the portion of the cathodecontaining the cathode mixture. This extending portion of the currentcollector can provide a convenient area for making contact with theelectrical lead connected to the positive terminal. It is desirable tokeep the volume of the extending portion of the current collector to aminimum to make as much of the internal volume of the cell available foractive materials and electrolyte.

FeS₂ cathodes may be made by roll coating a slurry of active materialmixture materials in a highly volatile organic solvent (e.g.,trichloroethylene) onto both sides of a sheet of aluminum foil, dryingthe coating to remove the solvent, calendering the coated foil tocompact the coating, slitting the coated foil to the desired width, andcutting strips of the slit cathode material to the desired length. It isdesirable to use cathode materials with small particle sizes to minimizethe risk of puncturing the separator.

The cathode is electrically connected to the positive terminal of thecell. This may be accomplished with an electrical lead, often in theform of a thin metal strip or a spring, as shown in FIG. 1. The lead isoften made from nickel plated stainless steel.

The separator may be a thin microporous membrane that is ion-permeableand electrically nonconductive. It is capable of holding at least someelectrolyte within the pores of the separator. The separator is disposedbetween adjacent surfaces of the anode and cathode to electricallyinsulate the electrodes from each other. Portions of the separator mayalso insulate other components in electrical contact with the cellterminals to prevent internal short circuits. Edges of the separatoroften extend beyond the edges of at least one electrode to insure thatthe anode and cathode do not make electrical contact even if they arenot perfectly aligned with each other. It may be desirable to minimizethe amount of separator extending beyond the electrodes.

To provide good high power discharge performance it may be desirablethat the separator have the characteristics (pores with a smallestdimension of at least 0.005 μm and a largest dimension of no more than 5μm across, a porosity in the range of 30 to 70 percent, an area specificresistance of from 2 to 15 ohm-cm² and a tortuosity less than 2.5)disclosed in U.S. Pat. No. 5,290,414, issued Mar. 1, 1994, and herebyincorporated by reference. Suitable separator materials should also bestrong enough to withstand cell manufacturing processes as well aspressure that may be exerted on the separator during cell dischargewithout tears, splits, holes or other gaps developing that could resultin an internal short circuit.

To minimize the total separator volume in the cell, the separator shouldbe as thin as possible, but at least about 1 μm or more so a physicalbarrier is present between the cathode and anode to prevent internalshort circuits. That said, the separator thickness may range from about1 to about 50 μm, desirably from about 5 to about 25 μm, and preferablyfrom about 10 to about 16 or about 20 μm. The required thickness willdepend in part on the strength of the separator material and themagnitude and location of forces that may be exerted on the separatorwhere it provides electrical insulation.

A number of characteristics besides thickness can affect separatorstrength. One of these is tensile stress. A high tensile stress isdesirable, such as, for example, at least 800 kilograms of force persquare centimeter (kgf/cm²), and desirably at least 1000 (kgf/cm²).Because of the manufacturing processes typically used to makemacroporous separators, tensile stress is typically greater in themachine direction (MD) than in the transverse direction (TD). Theminimum tensile stress required can depend in part on the diameter ofthe cell. For example, for a FR6 type cell the preferred tensile stressis at least 1500 kgf/cm² in the machine direction and at least 1200kgf/cm² in the transverse direction, and for a FRO3 type cell thepreferred tensile strengths in the machine and transverse directions are1300 and 1000 kgf/cm², respectively. If the tensile stress is too low,manufacturing and internal cell forces can cause tears or other holes.In general, the higher the tensile stress the better from the standpointof strength. However, if the tensile stress is too high, other desirableproperties of the separator may be adversely affected.

Tensile stress can also be expressed in kgf/cm, which can be calculatedfrom tensile stress in kgf/cm² by multiplying the latter by theseparator thickness in cm. Tensile stress in kgf/cm is also useful foridentifying desirable properties related to separator strength.Therefore, it may be desirable that the separator have a tensile stressof at least 1.0 kgf/cm, preferably at least 1.5 kgf/cm and morepreferably at least 1.75 kgf/cm in both the machine and transversedirections. For cells with diameters greater than about 0.45 inch (11.4mm), a tensile stress of at least 2.0 kgf/cm is most preferable.

Another indicator of separator strength is its dielectric breakdownvoltage. Preferably the average dielectric breakdown voltage will be atleast 2000 volts, more preferably at least 2200 volts. For cylindricalcells with a diameter greater than about 0.45 in (11.4 mm), the averagedielectric breakdown voltage is most preferably at least 2400 volts. Ifthe dielectric breakdown voltage is too low, it is difficult to reliablyremove cells with defective or damaged separators by electrical testing(e.g., retention of a high voltage applied to the electrode assemblybefore the addition of electrolyte) during cell manufacturing. It isdesirable that the dielectric breakdown is as high as possible whilestill achieving other desirable separator properties.

The average effective pore size is another of the more importantindicators of separator strength. While large pores are desirable tomaximize ion transport through the separator, if the pores are too largethe separator will be susceptible to penetration and short circuitsbetween the electrodes. The preferred maximum effective pore size isfrom 0.08 μm to 0.40 μm, more preferably no greater than 0.20 μm.

The BET specific surface area is also related to pore size, as well asthe number of pores. In general, cell discharge performance tends to bebetter when the separator has a higher specific surface area, but theseparator strength tends to be lower. It is desirable for the BETspecific surface area to be no greater than 40 m²/g, but it may bedesirable that it be at least 15 m²/g, or at least 25 m²/g.

A low area specific resistance may be desirable for good high rate andhigh power cell discharge performance. Thinner separators tend to havelower resistances, but the separator should also be strong enough,limiting how thin the separator can be. Desirably the area specificresistance is no greater than 4.3 ohm-cm², more preferably no greaterthan 4.0 ohm-cm², and most preferably no greater than 3.5 ohm-cm².

Separator membranes for use in lithium batteries are often made ofpolypropylene, polyethylene or ultrahigh molecular weight polyethylene,with polyethylene being preferred. The separator can be a single layerof biaxially oriented microporous membrane, or two or more layers can belaminated together to provide the desired tensile strengths inorthogonal directions. A single layer may help minimize the cost.Suitable single layer biaxially oriented polyethylene microporousseparator is available from Tonen Chemical Corp., available from EXXONMobile Chemical Co., Macedonia, N.Y., USA. Setela F20DHI grade separatorhas a 20 μm nominal thickness, and Setela 16 MMS grade has a 16 μmnominal thickness.

The anode, cathode, and separator strips are combined together in anelectrode assembly. The electrode assembly may be a spirally wounddesign, such as that shown in FIG. 1, made by winding alternating stripsof cathode, separator(s), anode, and separator around a mandrel, whichis extracted from the electrode assembly when winding is complete. Atleast one layer of separator and/or at least one layer of electricallyinsulating film (e.g., polypropylene) is generally wrapped around theoutside of the electrode assembly. This serves a number of purposes: ithelps hold the assembly together and may be used to adjust the width ordiameter of the assembly to the desired dimension. The outermost end ofthe separator or other outer film layer may be held down with a piece ofadhesive tape or by heat sealing.

Rather than being spirally wound, the electrode assembly may be formedby folding the electrode and separator strips together. The strips maybe aligned along their lengths and then folded in an accordion fashion,or the anode and one electrode strip may be laid perpendicular to thecathode and another electrode strip and the electrodes alternatelyfolded one across the other (orthogonally oriented), in both casesforming a stack of alternating anode and cathode layers.

The electrode assembly is inserted into the housing container. In thecase of a spirally wound electrode assembly, whether in a cylindrical orprismatic container, the major surfaces of the electrodes areperpendicular to the side wall(s) of the container (in other words, thecentral core of the electrode assembly is parallel to a longitudinalaxis of the cell). Folded electrode assemblies are typically used inprismatic cells. In the case of an accordion-folded electrode assembly,the assembly is oriented so that the flat electrode surfaces at oppositeends of the stack of electrode layers are adjacent to opposite sides ofthe container. In these configurations the majority of the total area ofthe major surfaces of the anode is adjacent the majority of the totalarea of the major surfaces of the cathode through the separator, and theoutermost portions of the electrode major surfaces are adjacent to theside wall of the container. In this way, expansion of the electrodeassembly due to an increase in the combined thicknesses of the anode andcathode is constrained by the container side wall(s).

A nonaqueous electrolyte, containing water only in very small quantitiesas a contaminant (e.g., no more than about 500 parts per million byweight, depending on the electrolyte salt being used), is used in thebattery cell of the invention. Any nonaqueous electrolyte suitable foruse with lithium and active cathode material may be used. Theelectrolyte contains one or more electrolyte salts dissolved in anorganic solvent. For a Li/FeS₂ cell, examples of suitable salts includelithium bromide, lithium perchlorate, lithium hexafluorophosphate,potassium hexafluorophosphate, lithium hexafluoroarsenate, lithiumtrifluoromethanesulfonate, and lithium iodide; and suitable organicsolvents include one or more of the following: dimethyl carbonate,diethyl carbonate, methylethyl carbonate, ethylene carbonate, propylenecarbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, methylformate, γ-butyrolactone, sulfolane, acetonitrile,3,5-dimethylisoxazole, n,n-dimethyl formamide, and ethers. Thesalt/solvent combination will provide sufficient electrolytic andelectrical conductivity to meet the cell discharge requirements over thedesired temperature range. Ethers are often desirable because of theirgenerally low viscosity, good wetting capability, good low temperaturedischarge performance and good high rate discharge performance. This isparticularly true in Li/FeS₂ cells because the ethers are more stablethan with MnO₂ cathodes, so higher ether levels can be used. Suitableethers include, but are not limited to acyclic ethers such as1,2-dimethoxyethane, 1,2-diethoxyethane, di(methoxyethyl)ether,triglyme, tetraglyme, and diethyl ether; and cyclic ethers such as1,3-dioxolane, tetrahydrofuran, 2-methyl tetrahydrofuran, and3-methyl-2-oxazolidinone. A particularly suitable non-aqueouselectrolyte is an electrolyte comprising lithium iodide in a solventcomprising at least one ether as disclosed in U.S. Pat. No. 5,514,491,the entire disclosure of which is incorporated herein by reference.

Accordingly, various combinations of electrolyte salts and organicsolvents can be utilized to form the electrolyte for electrochemicalcells. The molar concentration of the electrolyte salt can be varied tomodify the conductive properties of the electrolyte. Examples ofsuitable nonaqueous electrolytes containing one or more electrolytesalts dissolved in an organic solvent include, but are not limited to, a1 mole per liter solvent concentration of lithiumtrifluoromethanesulfonate (14.60% by weight) in a solvent blend of1,3-dioxolane, 1,2-diethoxyethane, and 3,5-dimethyl isoxazole(24.80:60.40:0.20% by weight), which has a conductivity of 2.5 mS/cm; a1.5 moles per liter solvent concentration of lithiumtrifluoro-methanesulfonate (20.40% by weight) in a solvent blend of1,3-dioxolane, 1,2-diethoxyethane, and 3,5-dimethylisoxazole(23.10:56.30:0.20% by weight), which has a conductivity of 3.46 mS/cm;and a 0.75 mole per liter solvent concentration of lithium iodide (9.10%by weight) in a solvent blend of 1,3-dioxolane, 1,2-diethoxyethane, and3,5-dimethylisoxazole (63.10:27.60:0.20% by weight), which has aconductivity of 7.02 mS/cm. Electrolytes utilized in the electrochemicalcells of the present invention have conductivity generally greater thanabout 2.0 mS/cm, desirably greater than about 2.5 or about 3.0 mS/cm,and preferably greater than about 4, about 6, or about 7 mS/cm.

Specific anode, cathode, and electrolyte compositions and amounts can beadjusted to provide the desired cell manufacturing, performance, andstorage characteristics. For example, the cell may be designed toprovide an anode to cathode input ratio of less than 1.0, equal to 1.0,or greater than 1.0. A cell with an anode to cathode input ratio of lessthan 1.0 may be said to have an anode under-balance, and a cell with ananode to cathode input ratio of greater than 1.0 may be said to have ananode over-balance. It may be desirable to provide a cell having ananode to cathode input ratio of less than or equal to 1.0. As usedherein, the anode to cathode input ratio may be calculated as follows:

Anode Capacity Per Linear Inch:

(foil thickness)×(interfacial electrode width)×1 inch×(density oflithium foil at 20° C.)×(lithium energy density, 3861.7 mAh/gm).

Cathode Capacity Per Linear Inch:

(final cathode coating thickness)×(interfacial electrode width)×1inch×(cathode dry mix density)×(final cathode packing percentage)×(dryweight percent FeS₂)×(percent purity FeS₂)×(FeS₂ energy density, 893.58mAh/gm)

Anode/cathode input ratio=anode capacity per linear inch/cathodecapacity per linear inch

“Interfacial electrode width” as used herein is the linear dimensionthat shares an interfacial area between the cathode and the anode.“Final cathode coating thickness” refers to the coating thickness afterany calendering operation or other densification processing of thecathode. “Final cathode packing percentage” refers to the solid volumepercentage after any calendering operation or other densificationprocessing and is equivalent to 100 percent less the void volumepercentage after any calendering operation or other densificationprocessing of the cathode. The “cathode dry mix density” refers to theadditive density of the solid components of the cathode coating.

The cell can be closed and sealed using any suitable process. Suchprocesses may include, but are not limited to, crimping, redrawing,colleting, and combinations thereof. For example, for the cell in FIG.1, a bead is formed in the can after the electrodes and insulator coneare inserted, and the gasket and cover assembly (including the cellcover, contact spring and vent bushing) are placed in the open end ofthe can. The cell is supported at the bead while the gasket and coverassembly are pushed downward against the bead. The diameter of the topof the can above the bead is reduced with a segmented collet to hold thegasket and cover assembly in place in the cell. After electrolyte isdispensed into the cell through the apertures in the vent bushing andcover, a vent ball is inserted into the bushing to seal the aperture inthe cell cover. A PTC device and a terminal cover are placed onto thecell over the cell cover, and the top edge of the can is bent inwardwith a crimping die to hold the gasket, cover assembly, PTC device andterminal cover and complete the sealing of the open end of the can bythe gasket.

The above description is particularly relevant to cylindrical Li/FeS₂cells, such as FR6 and FRO3 types, as defined in International StandardsIEC 60086-1 and IEC 60086-2, published by the InternationalElectrotechnical Commission, Geneva, Switzerland. However, the inventionmay also be adapted to other cell sizes and shapes and to cells withother electrode assembly, housing, seal and pressure relief ventdesigns.

Features of the invention and advantages thereof are further illustratedin the following examples:

EXAMPLES Comparative Example 1

Synthetic FeS₂ is prepared by reacting Fe₂O₃ and excess hydrogen sulfide(H₂S) at 300-400° C. for 8-24 hours as described by Tamura et al(Electrochimica Acta, 28 (1983) page 269). In a typical reaction, 5 g ofoxide was loaded into a porcelain boat that was subsequently loaded intoa glass tube. The tube was placed in a high temperature furnace. Theatmosphere in the tube was purged with argon before starting thehydrogen sulfide. The furnace was then heated to temperature and H₂Sallowed to continually flow throughout the reaction time. At the end ofthe reaction, the tube was again purged with inert gas and cooled. FIG.2 is an X-ray diffraction pattern of the product produced in thisComparative Example. As shown in FIG. 2, the reaction produced FeS₂ asshown by the peaks at 43°, 50°, 56°, 62°, 73°, 89°, 94°, 99°, and 104°.The X-ray diffraction, however, shows that the process from this examplealso produced FeS as evidenced by the peaks at 45°, 51.5°, 68°, 84°, and91.5°.

Example 1 Sulfidation Process

Synthetic FeS₂ is prepared using a sulfidation process in accordancewith the present invention as follows: 2.6 grams of nanorust (nanoparticles of Fe₂O₃) from Alfa Aesar having an average particle size ofabout 3 nm and 0.5 grams of elemental sulfur from Alfa Aesar having aparticle size of about 1-2 μm are charged to a flask as part of aLabconco rotary evaporator. The system is purged with argon before theintroduction of H₂S is started. The flask is heated to the desiredtemperature using an oil bath. Hydrogen sulfide gas (about 6% volumepercent in N₂) is flowed into the system. The hydrogen sulfide flow maybe started before heating or after the oil bath had reached the desiredtemperature (125-200° C.). After the appropriate exposure time ofhydrogen sulfide, about 5-6 hours for the solid masses listed above, theflask was raised out of the oil bath and the head pressure bled off andswitched over to argon. When the flask and contents were cool, it wascapped and quickly transferred to a drybox.

FIG. 3 is an X-ray diffraction pattern of the synthetic FeS₂ prepared inaccordance with Example 1. As shown in FIG. 3, the product from asulfidation process in accordance with the present invention providesFeS₂ having a pyrite crystal phase, as evidenced by the peaks at 43°,50°, 56°, 62°, 73°, 89°, 94°, 99°, and 104° using Cr radiation. FIG. 3also shows the presence of some marcasite crystals in the FeS₂ productas evidenced by the peaks at 39°, 59°, and 81°. Pyrite and marcasiteshare a peak at 50°. As shown in FIG. 3, the product does not containany FeS.

FIG. 4 is a moderate magnification SEM image of synthetic FeS₂ preparedin accordance with Example 1. As shown in FIG. 4, the particles appearto have a particle size in the range of from about 30 to about 60 nm.FIG. 5 is a field emission SEM (FESEM) image of FeS₂ prepared inaccordance with Example 1. As shown in FIG. 5, in some instances, theparticles appear to be formed by an agglomeration of severalcrystallites having a crystallite size of about 10 nm to about 15 nm.

The physical properties of the synthetic FeS₂ and the natural FeS₂ arecompared in Table 1. The average BET surface area of the FeS₂ particlesin this Example is about 105 m²/g.

TABLE 1 Synthetic FeS₂ Property (Example 1) Natural FeS₂ Particle Size(nm) 30-60 19,000 Crystallite Size (nm) 10 — BET Surface Area (m²/g) 1050.7 Neutron Activation 0.93 1.54 (% Oxygen) Number of Trace 0 11Metals >1000 ppm (Al, As, Ca, Co, Cu, K, Mg, Mn, Pb, Si, Zn)

Example 2 Sulfidation Process

Synthetic FeS₂ of Example 1 is sintered at 462° C. for two hours.Sintering causes the FeS₂ particles to grow and produce FeS₂ particleshaving a particle size of about 150 nm and a crystallite size of about73 nm. FIG. 6 illustrates the X-ray diffraction pattern of the sinteredFeS₂. In FIG. 6, the X-ray diffraction pattern of the FeS₂ from Example1 (see FIG. 3) is superimposed over the X-ray diffraction pattern of thesintered FeS₂. The sintered FeS₂ sample is represented by the patternhaving the sharper, more intense peaks. In FIG. 6, the asterisk symbolsby the peaks at 30°, 59°, and 81° in the pattern for the FeS₂ product ofExample 1 indicate the presence of marcasite in the unsintered product.As shown in FIG. 6, the sintered FeS₂ does not exhibit any peaksattributable to marcasite crystals. Thus, the marcasite crystals appearto have been converted to pyrite crystals.

Electrical performance of the synthetic FeS₂ prepared in Example 2 isanalyzed using the ANSI digital still camera (DSC) test method. The testis run, at room temperature, as follows. In a test vehicle scaled downrelative to a full AA cell and containing a scaled amount of FeS₂ asactive material, 1.5 W is applied for 2 seconds followed by 0.65 W for28 seconds. This cycle is repeated nine more times. The cell is thenallowed to recover under no load for 55 minutes before the whole processis repeated. This nested loop is repeated to some low voltage. The totalminutes under load to a 1.05V cutoff are reported. The amount of activeFeS₂ in the cell is about 18-20 mg for tests evaluating natural pyrite,and about 7-10 mg for tests evaluating the synthetic FeS₂. FIG. 7 showsthe voltage discharge characteristics of a natural FeS₂ sample employedin typical Energizer factory product and synthetic FeS₂ from Example 2.As shown in FIG. 7, the difference between the 55 minute rest OCV andthe high power result is around 300 mV for slightly more than half thetest time (around 40 hours) and then gradually increases to 400 mV bythe cut voltage of 1.05V. In the cells using the synthetic FeS₂ preparedin accordance with Example 2, the polarization is in the low 200 mVrange for most of the test but doesn't begin to increase until a testtime around 60 hours and doesn't increase to a final polarization of 400mV until around 70 hours. The synthetic FeS₂ of Example 1 has an averagevoltage on rest of about 1.75V for about half the test, while thenatural FeS₂ averaged around 1.55V. FIG. 7 also shows that the syntheticFeS₂ of Example 2 mimics the known two plateau discharge seen at lowconstant current rates and/or at elevated temperatures.

FIG. 8 compares the discharge profile of natural FeS₂ and the syntheticFeS₂ of Example 2 at 20 mA constant current, and FIG. 9 includesdischarge profiles at 20 mA and 200 mA constant current. As shown inFIGS. 8 and 9, at room temperature, the synthetic FeS₂ of Example 1exhibits a two plateau discharge at low power (20 mA) and also appearsto exhibit a two plateau discharge at 200 mA.

Specific energy density results are derived from the discharge dataobtained at 200 mA. FIG. 10 compares the specific energy density valuesof the natural FeS₂ and the synthetic FeS₂ of Example 2. As shown inFIG. 10, only the synthetic FeS₂ of Example 2 has a significant energydensity above 1.4V.

Example 3 Sulfidation Process

Synthetic FeS₂ obtained from the process of Example 1 is sintered at700° C. for 2 days under vacuum at a pressure of about 10⁻⁷ torr toprovide synthetic FeS₂ particles having an average particle size ofabout 1 to about 2 μm.

Example 4 Sulfidation Process

Synthetic FeS₂ is prepared by a sulfidation process as described inExample 1 except that the reaction is carried out at temperature ofabout 200° C. The FeS₂ in this Example has an average particle size inthe range of from about 100 to about 150 nm.

Example 5 Milling Process

Synthetic FeS₂ is prepared by a milling process as follows: Sulfurpowder and iron powder are charged to a SPEX vial in about a 2:1 molarratio of sulfur to iron. The total amount of iron and sulfur powder isabout 13 grams. Carbon steel balls, which are used as the milling media,are also charged to the vial. The total weight of the milling media isabout 89 grams. 13 grams of hexane, which is utilized as the processcontrol agent, is charged to the vial under an argon atmosphere. Thepowders are mechanical milled for about five hours to provide a powdermixture. After milling, the vial is opened in a glove box (inert Aratmosphere) and the hexane is allowed to evaporate.

After the hexane evaporates, the powder mixture is vacuum encapsulatedin quartz and annealed to form FeS₂. The powder mixture is annealed byheating at a temperature of 450° C.; the mixture is heated by increasingthe temperature 2° C. per minute up to 450° C. and holding thetemperature at 450° C. for forty-five minutes to form FeS₂.

FIG. 11 is an X-ray diffraction pattern of the product formed in thisExample. As shown in FIG. 11, the product is FeS₂ having a pyritecrystal phase, as evidenced by the peaks at 43°, 50°, 56°, 62°, 73°,89°, 94°, 99°, and 104°. The X-ray diffraction pattern also shows asmall peak at 68°, which may be attributed to some FeS in the product.Such an impurity may be due to a small excess of iron in the system fromthe steel milling media.

FIG. 12 is a SEM image of the particles produced in accordance with thisExample. The particles have an average particle size of about 2-3 μm anda crystallite size of about 160 nm.

FIG. 13 is a SEM image of a cross-section of FeS₂ particles produced inaccordance with this Example. FIG. 13 shows that the particles possesssome void volume and, therefore, exhibit some porosity. The FeS₂ of thisExample has a BET surface area of about 2.7 m²/g. the FeS₂ also has acarbon content of about 0.15% by weight of the FeS₂.

Example 6 Sulfidation Process

Synthetic FeS₂ is prepared using a sulfidation process as follows: 17.5grams of nanorust (nano particles of Fe₂O₃) from Alfa Aesar having anaverage particle size of about 3 nm and 3.5 grams of elemental sulfurfrom Alfa Aesar having a particle size of about 1-2 μm are charged to aflask. The system is purged with argon before the introduction of H₂S isstarted. The flask is heated using an oil bath. A flow of hydrogensulfide gas (about 40 volume percent H₂S in N₂) is started at the sametime as the heating of the oil bath. The reaction is allowed to proceedfor about an hour at 125° C. before ramping up to the final desiredtemperature (e.g., about 200° C.). After the appropriate exposure timeof hydrogen sulfide, about 5 hours for the solid masses listed above,the flask is raised out of the oil bath and the head pressure is bledoff and switched over to argon. When the flask and contents are cool, itis capped and transferred to a drybox.

X-ray diffraction of the product material showed peaks consistent withFeS₂ pyrite (˜43°, 50°, 56°, 62°, 73°, 89°, 94°, 99°, and 104° using Crradiation) and FeS₂ marcasite (˜39°, 50°, and)59°.

While the present invention has been described herein with reference tovarious exemplary embodiments thereof, the invention is not intended tobe limited to such embodiments. Further, upon reading and understandingthe present application, modifications and changes may occur to thoseskilled in the art without departing from the spirit of the disclosedtechnology. It is intended that the disclosed technology be consideredas including all such modifications and changes.

1. A method for forming FeS₂ comprising: intimately mixing iron powderand sulfur powder in the presence of a process control agent and amilling media to form a substantially homogeneous powder mixture, andannealing the powder mixture for a sufficient period of time to formFeS₂.
 2. The method according to claim 1, wherein annealing the powdermixture comprises heating the powder mixture at a temperature of atleast about 400° C. to a temperature below about 740° C.
 3. The methodaccording to claim 1, wherein annealing the powder mixture comprisesheating the powder mixture at a temperature of at least about 450° C. toa temperature below about 500° C.
 4. The method according to claim 1,wherein annealing the powder mixture comprises raising the temperatureat a rate of about 1 to about 3 degrees per minute until the desiredannealing temperature is reached and holding the temperature at thedesired annealing temperature.
 5. The method according to claim 1,wherein the annealing temperature is achieved by raising the temperatureat a rate of about 1 to about 3 degrees per minute through the meltingpoint of sulfur, and subsequently raising the temperature to about 450°C. at a rate greater than about 3 degrees per minute.
 6. The methodaccording to claim 1, wherein the process control agent comprises ahydrocarbon and is substantially free of an oxygen moiety.
 7. The methodaccording to claim 6, wherein the process control agent comprisespentane, heptane, hexane, octane, nonane, decane, or a combination oftwo or more thereof.
 8. The method according to claim 1, wherein theamount of process control agent initially charged to the system is fromabout 5 to about 12 percent by weight of the total weight of ironpowder, sulfur powder, and milling media.
 9. The method according toclaim 1, wherein the iron powder and sulfur powder are provided in a 1:2molar ratio of iron to sulfur.
 10. The method according to claim 1,wherein the milling media is chosen from a steel media, ceramic media, aglass media, a zirconia media, a tungsten/cobalt media, or mixtures oftwo or more thereof.
 11. The method according to claim 1, wherein themilling media comprises a steel media.
 12. The method according to claim11, wherein the sulfur powder is provided in an amount in excess of thestoichiometric amount required for forming FeS₂.
 13. The methodaccording to claim 1, wherein the weight ratio of iron powder and sulfurpowder to milling media is in the range of from about 1:4 to about 1:10.14. The method according to claim 1, further comprising milling a dopantwith the iron powder and the sulfur powder.
 15. The method according toclaim 14, wherein the dopant comprises a conductive material chosen fromat least one metal, graphite, carbon black, or a combination of two ormore thereof.
 16. The method according to claim 1, wherein the FeS₂comprises carbon entrained from the process control agent.
 17. Themethod according to claim 16, wherein the FeS₂ comprises about 1% byweight or less of carbon.
 18. The method according to claim 16, whereinthe FeS₂ comprises about 0.15% by weight or less of carbon. 19.Synthetic FeS₂ formed by the method according to claim
 1. 20. A methodfor forming FeS₂ comprising: performing a first milling operationcomprising intimately mixing iron powder and sulfur powder in thepresence of a process control agent and a milling media t form asubstantially homogeneous powder mixture; removing the process controlagent; and performing a second milling operation comprising milling thehomogeneous powder mixture for a sufficient period of time to form FeS₂.21. Synthetic FeS₂ formed by the method according to claim 20.